A rotation angle of a rotor needs to be detected in order to control a rotational position of a motor. A rotation angle of the rotor of the motor can be generally detected by connecting a rotary encoder with a rotation axis of the rotor of the motor. In this case, the rotary encoder outputs a two phase pulse signal, varying according to a rotation angle of the rotor of the motor, and having ¼ cycle phase difference. A relative rotation angle can be detected based on edge detection and two phase High/Low status of an output signal of the rotary encoder.
An optical encoder, as an above mentioned rotary encoder, is composed of a disk, on which slits are formed at equal intervals in its outer peripheral part, and two photo interrupters disposed at ¼ intervals of a slit pitch of the disk. Then, the two phase pulse signal can be obtained by performing binarization of an output signal of the two photo interrupters.
Also, in a motor drive controlling device, described in Japanese laid open Patent Publication No. 2013-99023, a method for detecting and outputting a phase information signal, by using a plurality of sensor signals having signal levels corresponding to rotational positions of a rotor of a motor, is disclosed. That is, the sensor signals or corresponding signals U1, V1, and W1 are divided into a plurality of phase sections, compared with threshold levels corresponding to the phases, and it is detected that a signal level has reached the threshold to output a phase information signal Phsyn, indicating the detected phase.